Current theories of alcoholism posit that alcohol-induced neuroadaptations within limbic structures in the brain, including the nucleus accumbens (NAC), contribute to the transition from recreational alcohol drinking to excessivealcohol consumption. Recently, the Group 1 metabotropic glutamate receptor (mGluR)- associated scaffolding protein Homer2 was identified as an active and necessary cellular mediator of alcohol-induced neural plasticity in mice. Constitutively expressed Homer proteins facilitate Group 1 mGluR- stimulated intracellular signaling and cluster Group 1 mGluRs within the postsynaptic density, co-localizing these receptorswith other proteins implicated in synaptic plasticity, such as PI3K (phosphatiylionsitol-3 kinase). Homer2 deletion reduces the function and the expression of Group 1 mGluRs in the NAC in vivo and the alcohol-avoiding and -intolerant behavioral phenotype of Homer2 knock-out (KO) mice resembles that produced by the pharmacological blockade of Group 1 mGluRs. Collectively, these observations suggest that Group 1 mGluR-Homer signaling is an important cellular mediator of excessive alcohol consumption. To test this hypothesis directly, this proposal will employ in vivo pharmacological and genetic approaches to characterizethe role for Group 1 mGluR-Homer signaling within the NAC in regulating excessive alcohol consumption within the scheduled high alcohol consumption (SHAC) murine model (Aim 1). Immunohistochemical and immunoblotting approaches will be employed to determine the role for Homer2 n regulating the effects of sustained, excessivealcohol consumption upon the synaptic architecture of NAC neurons, as well as the formation, subcellular localization and function of mGluR-Homer signaling complexes (Aim 2). Finally, to relate genetic variance in excessive alcohol drinking to mGluR-Homer-PI3K expression and signaling within the NAC, immunoblotting the total protein content and membrane localization of members of the mGluR-Homer-PI3K signaling cascade will be compared between mouse lines selectively bred for high SHAC and SLAC (Scheduled Low Alcohol Consumption) phenotypes. The results of these studies will further our understanding of the cellular mechanisms involved in regulating the transition from recreational to excessive alcohol drinking and provide greater insight into the etiology of alcoholism and its treatment.